This invention relates to an apparatus for controlling pressures within a fuel tank in accordance with an operating condition of an engine.
For example, Japanese Utility Model Unexamined Publication No. 56-90923 discloses a conventional construction in which a check valve is provided in a pipe connected between a fuel tank and a canister. When evaporated fuel is produced within the fuel tank and the pressure within the tank reaches a predetermined level, the check valve is opened to supply the evaporated fuel to the canister.
In this conventional construction, however, the pressure in the fuel tank at which the fuel tank is caused to communicate with the canister is set to a predetermined level, and therefore in the case where this predetermined pressure is high, a large amount of evaporated fuel may be discharged to the ambient atmosphere if a leakage accident occurs between the fuel tank and the check valve. In contrast, where the predetermined pressure is low, an amount of evaporated fuel supplied to the canister increases, which results in a problem that the canister must be increased in size.
Therefore, there has been proposed a technique in which there is set a predetermined pressure of a fuel tank, at which the fuel tank is caused to communicate with a canister, by an intake negative pressure when an engine is in a stopped condition, and when the pressure within the fuel tank exceeds this predetermined level, evaporated fuel is supplied to the canister. In an operating condition of the engine, the fuel tank is always in communication with the canister to supply the evaporated fuel to the canister.
In this prior art technique, however, since the fuel tank is always in communication with the canister in the operating condition of the engine, an amount of the evaporated fuel from the fuel tank is increased by the negative pressure from the engine. The evaporated fuel is not discharged to the ambient atmosphere, and therefore when a large amount of the evaporated fuel is supplied to the engine, the control of the air-fuel ratio is adversely affected. To overcome this problem, it is necessary to use the type of canister which positively adsorbs an increased amount of evaporated fuel; however, this increases the size of the canister.